Dual angle radiation scanning of objects

RELATED APPLICATION

0001The present application is a division of U.S. patent application Ser. No. 11/485,150, which was filed on Jul. 12, 2006, will issue on Mar. 20, 2012 bearing U.S. Pat. No. 8,137,976, and is incorporated by reference herein.

FIELD OF THE INVENTION

0002Radiation scanning of objects, including large objects such as cargo containers, to identify contraband.

BACKGROUND OF THE INVENTION

0003Radiation is commonly used in the non-invasive inspection of contents of objects, such as luggage, bags, briefcases, cargo containers, and the like, at airports, seaports, and public buildings, for example, to identify hidden contraband. The contraband may include hidden guns, knives, explosive devices, illegal drugs, and weapons of mass destruction, such as a nuclear or a “dirty” radioactive bomb, for example. During radiation scanning, an object is irradiated by a radiation beam. Radiation transmitted through the object, which is attenuated to varying degrees by the contents, dependent on the densities of the materials through which the radiation beam passes, is detected. Higher density materials or regions within the object attenuate radiation more than less dense materials, resulting in darker or lighter regions on an X-ray image, depending on how the detected radiation is processed and displayed.

0004One common inspection system is a line scanner, where the object to be inspected is passed between a stationary source of radiation, such as X-ray radiation, and a stationary detector. The radiation is collimated into a fan beam or a pencil beam. The radiation transmitted through the object is detected and measured. Radiographic images of the contents of the object may be generated for inspection. The images show the shape, size and varying densities of the contents.

0005Fissionable, fissile, and fertile materials (“nuclear materials”) have high atomic numbers (Z) and high densities. Their presence, therefore, causes high attenuation of a radiation beam passing therethrough. Fissile materials, such as uranium-235 (Z=92), uranium-233 (Z=92), and plutonium-239 (Z=94), may undergo fission by the capture of a slow (thermal) neutron. Fissionable materials include fissile materials, and materials that may undergo fission by capture of fast neutrons, such as uranium-238. Fertile materials may be converted into fissile materials by the capture of a slow (thermal) neutron. Uranium-238, for example, may be converted into plutonium-239. Thorium-232 (Z=90), for example, may be converted into uranium-233. Fissionable, fissile, and fertile material are referred to herein as “nuclear material.” Special Nuclear Material (“SNMs”), which more readily undergo fission than other fissile materials, are defined by the U.S. Nuclear Regulatory Commission to include plutonium, uranium-233, and uranium enriched in the isotopes of uranium-233 or -235. All of these materials have densities of about 20 g/cm³.

0006Radioactive materials, certain of which may have lower atomic numbers and densities than nuclear materials (cobalt-60, for example, has an atomic number of 27 and a density of about 9 g/cm³), are typically shielded during shipping by high atomic number materials, such as lead (Z=82) or tungsten (Z=74). Lead has a density of about 11 g/cm³and tungsten has a density of about 19 g/cm³. These shielding materials also cause high attenuation. Iron, which is a main material in a majority of industrial goods shipped in cargo conveyances, in contrast, which has an atomic number of 26 and a density of about 8 g/cm³, causes less attenuation. Agricultural goods, which may also be shipped in cargo conveyances, have even lower atomic numbers and densities. It is noted, however, that large amounts of even low density materials, such as agricultural goods, along a line of sight of a radiation beam, can also cause high attenuation of radiation.

0007While the smuggling of guns, explosives and other contraband onto planes in carry-on bags and in luggage has been a well known, ongoing concern, a less publicized but also serious threat is the smuggling of contraband across borders and by boat in large cargo containers. Only 2%-10% of the 17 million cargo containers brought to the United States by boat are inspected. “Checkpoint Terror”, U.S. News and World Report, Feb. 11, 2002, p. 52.

0008Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters), and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers and use of the term “cargo conveyance” encompasses cargo containers and pallets.

0009Atomic bombs and “dirty bombs,” which use a conventional explosion to disperse radioactive material over a wide territory, are examples of nuclear devices that may be smuggled in cargo conveyances and smaller objects. Radioactive, fissionable, fissile, and fertile materials that may be used to manufacture atomic devices, may also be similarly smuggled in such objects.

0010A variety of techniques are being used to locate nuclear devices, nuclear materials, and radioactive materials (that may not be nuclear materials), in cargo conveyances. Manual inspection of the contents of an object is too slow for regular use. Identification of radioactive materials and nuclear devices by passive inspection systems, such as a radiation detector, while faster, is difficult, because the dense materials absorb most of the photons they emit. Shielding material, such as lead or tungsten, may also be used to block the escape of radiation, preventing its detection. In addition, certain fissile materials, such as uranium-233, uranium-235, and plutonium-239 while radioactive, have exceedingly long half-lives (on the order of 10⁴-10⁸years). The count rate from spontaneous decays for such material is so low, that passive detection is not reliable. Also, a relatively small amount of radioactive material may be located within a large cargo conveyance. It is also difficult to distinguish nuclear devices and nuclear materials from other dense items that may be contained within the object by standard X-ray scanning at one or multiple radiation energies.

0011In one example of an X-ray scanning system, U.S. Pat. No. 5,524,133 discloses scanning systems for large objects, such as freight in a container or on a vehicle. In one embodiment, two stationary sources of X-ray radiation are provided, each emitting a beam that is collimated into a fan beam. The sources face adjacent sides of the freight and the fan beams are perpendicular to each other. A stationary detector array is located opposite each source, on opposite sides of the freight, to receive radiation transmitted through the freight. In addition, X-ray radiation of two different energies are emitted by each source. One energy is significantly higher than the other. For example, energies of 1 MeV and 5 or 6 MeV may be used. A ratio of the mean number of X-rays detected at each energy endpoint by the detector array as a whole for each slice or by the individual detectors of the array is determined and compared to a look up table to identify a mean atomic number corresponding to the ratio. The material content of the freight is thereby determined.

0012One complication with the use of such X-ray scanning devices is that measurements of radiation after interaction with the object under inspection are statistical. The accuracy of a measurement of X-ray radiation transmitted through an object is limited by the number of photons used to make the measurement, as well as intrinsic system noise, for example. Repeated measurements of the same quantity typically yield a cluster of measurement values around a mean value. A plot of the cluster of measurements typically forms a “normal distribution” curve. The dispersion of the individual measurements (the width of the normal distribution curve) is characterized by a standard deviation. Insufficient photons may be detected to enable measurement distributions with small standard deviations. The distributions for materials of interest, such as uranium, may therefore overlap the distributions of other, non-threatening materials. It may not, therefore, be clear whether a particular measurement is indicative of a material of interest or not, resulting in either a high false positive rate or a low sensitivity.

0013The standard deviation decreases and the accuracy of measurement increases as more photons are detected. While the number of photons detected may be increased by increasing the scanning time, it is generally not acceptable to slow the throughput rate of a typical X-ray scanning system at ports, borders, or airports, for example.

0014The accuracy of a scanning system seeking to identify a material, such as uranium, for example, may be characterized by its “sensitivity” and its “specificity”. Sensitivity is the probability that the presence of uranium in a cargo conveyance will be identified. A system with high sensitivity will identify more true positives (correct identification of the presence of uranium) and fewer false negatives (missed detection of uranium) than a system with low sensitivity. However, increased sensitivity may result in an increase in the number of false positives, which may not be acceptable. Specificity, which is a statistical measure of accuracy, is the probability that the scanning system will properly identify the absence of uranium in a cargo conveyance, for example. A system with high specificity will identify fewer false positives (identification of uranium in a cargo conveyance when it is not present), than a system with low specificity.

0015In U.S. Pat. No. 6,347,132 B1 (“Annis”), a high energy X-ray inspection system for detecting nuclear weapons materials is described wherein an object is scanned by a high energy X-ray fan beam or pencil beam. A detected signal is processed to identify an area of high X-ray attenuation within the object. Such high attenuation is considered to be indicative of the presence of nuclear materials.

SUMMARY OF THE INVENTION

0016Since an X-ray image is a two-dimensional representation of a three-dimensional space, it cannot be determined from the detected X-ray signal or from a resulting X-ray image whether a region of high density is due to attenuation caused by a small region of very high density, high atomic number material (“HANM”), along the line of sight of the X-ray radiation, which may be a nuclear material or shielding material for nuclear material, or attenuation caused by a larger region of moderate and/or low density, innocuous material or materials along the line of sight of the X-ray beam. For example, 5 cm of a material having a moderate density of about 8 g/cm³, such as iron, would cause about the same attenuation and would therefore appear to have about the same density in an X-ray image as 2 cm of the HANMs uranium or plutonium, which have densities of about 20 g/cm³. Large regions of moderate and/or low density materials will therefore cause false positives for nuclear/shielding material in X-ray images, that need to be further investigated. This causes unnecessary delays in processing of many cargo conveyances, luggage, etc.

0017To resolve this ambiguity without having to manually inspect the contents of an object, in embodiments of the present invention, objects may be scanned at least two different angles. In one example, if a high density region is found from scanning the object at the first angle, the object is scanned at a second angle. In this way, the contents of the object contributing to the radiation attenuation resulting in the high density region at the first angle, may be examined at the second angle. If scanning at the second angle also results in a high density region, it is more likely that HANM, which may be nuclear material or shielding material for HANM, is present in the object. If not, then it is not likely that nuclear material is present. The size of the object may optionally be taken into consideration, as well. For example, scanning at the second angle may only take place if a high density region greater than a predetermined size of concern is found at the first angle. Images may be visually examined and/or data underlying the images or the detected radiation itself may be automatically examined.

0018In accordance with an embodiment of the invention, a method of examining contents of an object is disclosed comprising scanning at least a portion of an object by a first radiation beam at a first angle and detecting radiation interacting with the object at the first angle. The method further comprises scanning at least the portion of an object by a second radiation beam at a second angle different than the first angle and detecting radiation interacting with the object at the second angle. The method further comprises determining whether the object at least potentially contains high atomic number material based, at least in part, on the detected radiation at the first and second angles.

0019In accordance with another embodiment of the invention, a method of examining contents of an object is disclosed comprising scanning at least a portion of an object by a first X-ray radiation beam generated by a first X-ray source, at a first angle and detecting first radiation transmitted through the object by a first detector. The method further comprises determining whether the object at least potentially contains nuclear material based, at least in part, on the detected radiation. If it is determined that the object at least potentially contains nuclear material, the method further comprises scanning at least the portion of the object by a second X-ray radiation beam generated by a second X-ray source different than the first X-ray source, at a second angle different than the first angle, detecting second radiation transmitted through the object by a second detector, and determining whether the object at least potentially contains nuclear material based, at least in part, on the second detected radiation.

0020In accordance with another embodiment of the invention, a system for examining contents of an object is disclosed comprising means for scanning at least a portion of an object at first and second, different angles, means for detecting radiation interacting with the object at the first and second angles, and means for determining whether the object at least potentially comprises high atomic number material, based, at least in part, on the radiation detected at the first and second angles.

0021In accordance with another embodiment of the invention, a system for examining contents of an object is disclosed comprising at least one radiation source to scan at least a portion of an object at first and second, different angles and at least one detector positioned to detect radiation at the first and second angles. The system further comprises at least one processor coupled to the detector. The at least one processor is configured to determine whether the object at least potentially comprises high atomic number material based, at least in part, on the radiation detected at the first and second angles.

BRIEF DESCRIPTION OF THE DRAWINGS

0022<figref idref="DRAWINGS">FIG. 1</figref> is a perspective view of an example of an object containing a sphere and a solid pipe, moving through an example of a prior art X-ray scanning system;

0023<figref idref="DRAWINGS">FIG. 2</figref> is a front view of the object moving through the X-ray scanning system of <figref idref="DRAWINGS">FIG. 1</figref>;

0024<figref idref="DRAWINGS">FIG. 3</figref> is an example of an X-ray image resulting from radiation scanning of the object by the X-ray scanning system of <figref idref="DRAWINGS">FIG. 1</figref>;

0025<figref idref="DRAWINGS">FIG. 4</figref> is a schematic representation of a front view of an example of a radiation scanning system in accordance with an embodiment of the invention;

0026<figref idref="DRAWINGS">FIG. 5</figref><i>a </i>is an example of an X-ray image of the contents of an object resulting from scanning by the radiation scanning system of <figref idref="DRAWINGS">FIG. 4</figref>, at a first angle;

0027<figref idref="DRAWINGS">FIG. 5</figref><i>b </i>is an example of an X-ray image of the contents of the object of <figref idref="DRAWINGS">FIG. 5</figref> at a second angle, where the contents do not include high atomic nuclear material;

0028<figref idref="DRAWINGS">FIG. 5</figref><i>c </i>is another example of an X-ray image of the object of <figref idref="DRAWINGS">FIG. 5</figref> at the second angle, where the contents include high atomic nuclear material;

0029<figref idref="DRAWINGS">FIG. 6</figref> is a graph of Physical Density (g/cm³) versus Atomic Number for metals up to an atomic number of 57 (indicated by black points) and certain other elements;

0030<figref idref="DRAWINGS">FIG. 7</figref> is an example of a method for examining objects in accordance with an embodiment of the invention;

0031<figref idref="DRAWINGS">FIG. 8</figref> is a schematic representation of another example of a radiation scanning system in accordance with an embodiment, in which a radiation source is movably supported on a rail;

0032<figref idref="DRAWINGS">FIG. 9</figref> is a schematic representation of another example of a radiation scanning system in accordance with an embodiment of the invention, wherein a radiation source and a detector are movably supported on a gantry;

0033<figref idref="DRAWINGS">FIG. 10</figref> is a schematic representation of another example of a radiation scanning system in accordance with an embodiment of the invention, in which a radiation source comprises two targets, to generate radiation to irradiate an object at two angles; and

0034<figref idref="DRAWINGS">FIG. 11</figref> is a perspective view of an example of a radiation source similar to that in <figref idref="DRAWINGS">FIG. 10</figref>, comprising a linear accelerator body aligned with one of two targets.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0035<figref idref="DRAWINGS">FIG. 1</figref> is a perspective view of an example of an object <b>10</b>, such as a rectangular cargo conveyance, containing a sphere <b>12</b> of a high density SNM, such as uranium-233 (density of about 20 g/cm³) and a solid pipe <b>14</b> of a moderate density material, such as iron (density of about 8 g/cm³), for example, moving through an example of an X-ray scanning system. The object <b>10</b> is on a conveyor <b>16</b>, moving to the right, as indicated by arrow A. <figref idref="DRAWINGS">FIG. 2</figref> is a front view of the object <b>10</b> on the conveyor <b>16</b>, also showing a radiation source <b>18</b> and a detector <b>20</b>. <figref idref="DRAWINGS">FIG. 3</figref> is an example of an X-ray image <b>22</b> resulting from radiation scanning of the object <b>10</b>, including the sphere <b>12</b> of high density material, indicated by a high density region <b>24</b>, and the iron pipe <b>14</b>, indicated by a high density region <b>26</b>. In this example, the pipe <b>14</b> has a length such that the radiation attenuation caused by the pipe is comparable to the attenuation caused by the sphere <b>12</b> of uranium. The two regions <b>24</b>, <b>26</b> are therefore indistinguishable in the image <b>22</b>.

0036In accordance with embodiments of the invention, objects are examined at least two angles by radiation scanning to identify materials that may be high density, high atomic number materials (“HANMs”) within the object. Such HANM may be a nuclear material or shielding for a nuclear material. In one example, if a high density region is identified in a first radiographic image derived from radiation scanning at a first angle, then a second radiographic image derived from radiation scanning of a portion of the object including the high density region at a second angle is examined. If the corresponding portion of the object appears to have high density at the second angle, as well, it is likely that an HANM, which may be a nuclear material or shielding for a nuclear material, is present. If not, then nuclear or shielding material is not likely to be present. The images may be visually analyzed or automatically analyzed by a processor. A processor may analyze the data underlying the image or the detected radiation, instead of or along with visual inspection. The processor may also determine the size of the high density region and take that into consideration. For example, the second image may only be examined if the high density region of the first image is greater than or equal to a predetermined minimum size of concern.

0037A class of materials to be defined as HANM may be chosen by an operator or supplier of the system, for example. In one example, this is done by selecting an atomic number at or above which materials will be considered HANM. For example, HANM may be defined by the system to include special nuclear materials (“SNMs”) by selecting an atomic number at least greater than or equal to the atomic number of uranium (Z=92), for example. HANM may be defined by the system to include SNMs and shielding material by selecting an atomic number at least greater than or equal to the atomic number of lead (Z=74), for example. HANM may be defined by the system to include materials having an atomic number greater than iron, for example by selecting an atomic number of at least 26. In another example, HANM are defined as materials having a density at or above a selected density, such as 10 g/cm³, for example.

0038<figref idref="DRAWINGS">FIG. 4</figref> is a schematic representation of a front view of an example of a radiation scanning system <b>100</b> in accordance with an embodiment of the invention. The system <b>100</b> comprises a first radiation source <b>102</b> and a second radiation source <b>104</b>, positioned to irradiate an object <b>106</b> on a conveyor <b>108</b> at two different angles with first and second radiation beams <b>103</b>, <b>105</b>, respectively. A first detector <b>110</b> faces the first source <b>102</b> and a second detector <b>112</b> faces the second source <b>104</b>, on an opposite side of the object <b>106</b>. The first and second detectors are coupled to analog-to-digital (“A/D”) circuitry <b>114</b>, <b>116</b>, respectively, which are coupled to a processor <b>118</b>. The processor <b>118</b> may be coupled to an image reconstruction system <b>120</b>, which is coupled to a display <b>122</b>. The processor <b>118</b> may also be directly coupled to the display <b>122</b>. The processor <b>118</b> and/or the image reconstruction system <b>120</b> may be part of one or more computers <b>124</b>. Multiple processors <b>118</b>, image reconstruction systems <b>120</b>, and computers <b>124</b> may be provided. An operator's console (not shown) is also typically provided to directly or indirectly control the operation of the system <b>100</b>.

0039Collimators <b>107</b>, <b>109</b> are coupled to the radiation sources <b>102</b>, <b>104</b>. The collimators <b>107</b>, <b>109</b> include slots (not shown), to collimate the X-ray radiation into a beam having a desired shape. In one example, the collimators <b>107</b>, <b>109</b> define fan beams. Alternatively, the slots <b>107</b>, <b>109</b> may define a cone beam, such as an asymmetric pyramidal cone beam, or a pencil beam. The collimators <b>107</b>, <b>109</b> may have other shapes, as well. The collimator <b>107</b> may define a radiation beam having a different shape than the collimator <b>109</b>. For example, the slot <b>107</b> may define a cone beam and the slot <b>109</b> may define a fan beam or a pencil beam. Use of a first, broader cone beam may enable more rapid scanning of the object <b>106</b>. If a suspect region is found, scanning at the second angle may be performed with a second fan beam or pencil beam. This could reduce the overall radiation exposure of the object <b>106</b>, as well.

0040If the radiation sources <b>102</b>, <b>104</b> are linear accelerators, they may be positioned so that their longitudinal axes <b>102</b><i>a</i>, <b>104</b><i>a</i>, respectively, are substantially perpendicular, as shown in <figref idref="DRAWINGS">FIG. 4</figref>. The central axes <b>103</b><i>a</i>, <b>105</b><i>a </i>of the radiation beams <b>103</b>, <b>105</b> may or may not be perpendicular to each other, as well. For example, in the example of <figref idref="DRAWINGS">FIG. 4</figref>, since the source <b>102</b> is positioned at about the same height as the conveyor <b>108</b>, the radiation beam <b>103</b> is not emitted symmetrically with respect to the longitudinal axis <b>102</b><i>a </i>of the source. The central axis <b>103</b><i>a </i>of the beam <b>103</b> is not, therefore, aligned with the longitudinal axis <b>102</b><i>a </i>of the source <b>102</b>. The central axis <b>105</b><i>a </i>of the beam <b>105</b>, in contrast, is aligned with the central axis <b>104</b><i>a </i>of the source.

0041The central axes <b>103</b><i>a</i>, <b>105</b><i>a</i>, intersect to form an angle α, as shown in <figref idref="DRAWINGS">FIG. 4</figref>. The angle α is an angle that provides sufficient additional information about a suspect high density region identified by scanning at the first angle, to determine whether the high density region is potentially due to HANM. In one example, that additional information is in a second dimension different than the dimension of the first scanning angle, to provide a “perspective” view of the suspect high density region, yielding additional information about the density and size of the suspect region. In one example, the angle α is at least 20° and not greater than 160°. In another example, the angle α is at least 30° and not greater than 150°. In another example, the angle α is at least 45° and not greater than 135°. In another example, the angle α is at least 60° and not greater than 120°. In another example, the angle α is about 90°.

0042The sources <b>102</b>, <b>104</b> may also be positioned and the radiation beams <b>103</b>, <b>105</b> collimated such that the central axes <b>103</b><i>a</i>, <b>105</b><i>a </i>lie in the same plane or in different planes, such as parallel planes. If the axes <b>103</b><i>a</i>, <b>105</b><i>a </i>are different, the first source <b>102</b> needs to be upstream of the second source <b>104</b> so that a longitudinal location of the object <b>106</b> passes the first source <b>102</b> before passing the second source <b>104</b>.

0043The conveyor <b>108</b> supports and conveys the object <b>106</b> through the scanning system <b>100</b>, between the X-ray sources <b>102</b>, <b>104</b> and the detectors <b>110</b>, <b>112</b>. The conveyor system <b>108</b> may be a mechanically driven conveyor belt, a track or mechanically driven rollers, for example. The conveyor <b>108</b> may be transparent to radiation, as is known in the art.

0044To examine objects <b>106</b> having a width “W” greater than about 5 feet (1.5 meters), such as larger cargo conveyances, the X-ray sources <b>102</b>, <b>104</b> may generate radiation having energy endpoints greater than about 1 MeV. 5 MeV or 9 MeV may be used, for example. In examining objects having a width W less than about 5 feet (1.5 meters), the X-ray sources <b>102</b>, <b>104</b> may generate energy in the keV range. For example, 600 keV may be used.

0045One or both of the radiation sources <b>102</b>, <b>104</b> may be sources of X-ray radiation, such as Bremsstrahlung radiation. For example, the sources <b>102</b>, <b>104</b> may be linear accelerators, such as a Linatron® Linear Accelerator (“Linatron®”), having an accelerating potential at an appropriate level or levels, available from Varian Medical Systems, Inc., Palo Alto, Calif. (“Varian”), for example. In the Varian Linatron®, 360 pulses are output per second. The Varian Linatron® has an opening angle of about 20-30 degrees, for example. The X-ray sources <b>102</b>, <b>104</b> may be other types of sources, such as electrostatic accelerators, microtrons and betatrons, for example. The source or sources <b>102</b>, <b>104</b> may also include carbon-12 (C-12), cobalt-60 (Co-60), plutonium-beryllium (Pu—Be), and/or americium-beryllium (Am—Be) based sources. One or both of the radiation sources <b>102</b>, <b>104</b> may also be a source of neutrons. X-ray tubes may be used as radiation sources <b>102</b>, <b>104</b> in the KeV range.

0046The first and second detectors <b>110</b>, <b>112</b> collect radiation interacting with the object <b>106</b>. In the example of <figref idref="DRAWINGS">FIG. 4</figref>, radiation transmitted through the object <b>106</b> is detected. Scatter imaging could also be used, in addition to transmission imaging. Scatter imaging could be used instead of or in addition to transmission imaging. Scatter imaging could be used exclusively with smaller objects, such as luggage, for example. The detectors <b>110</b>, <b>112</b>, or other detectors, could be suitably positioned to detect scattered radiation, as is known in the art.

0047The detectors <b>110</b>, <b>112</b> may be in the form of a detector array. The configuration of the detectors <b>110</b>, <b>112</b> may depend on the shape of the respective collimated radiation beams <b>103</b>, <b>105</b>. For example, if either or both of the radiation beams <b>103</b>, <b>105</b> are collimated into fan beams, the corresponding detector arrays <b>110</b>, <b>112</b> may comprise one-dimensional detector arrays, comprising a single row of detector elements. If either or both of the collimated radiation beams are cone beams, such as asymmetric pyramidal cone beams, the corresponding detector arrays <b>110</b>, <b>112</b> may comprise two dimensional detector arrays <b>110</b>, <b>112</b> comprising two or more adjacent rows of detector elements. Either or both of the detector arrays <b>110</b>, <b>112</b> may comprise a plurality of modules of detectors, each comprising one or more rows of detector elements supported in a housing, as is known in the art. Either or both of the detectors <b>110</b>, <b>112</b> or detector arrays may be straight, as shown in <figref idref="DRAWINGS">FIG. 4</figref>, or L-shaped, as is known in the art. If either or both of the radiation beams <b>103</b>, <b>105</b> are collimated into pencil beams, a one-dimensional detector may be used.

0048The detectors <b>110</b>, <b>112</b> may be photon detectors, such as a photodiode detector array comprising inorganic scintillators, as is known in the art. Cadmium tungstate (CdWO₄) scintillators may be used, for example. Amorphous silicon (aSi) detectors, such as PaxScan™ detectors available from Varian Medical Systems, Inc., Palo Alto, Calif., may also be used.

0049The values of the intensities of the radiation transmitted through the object and by the detected detectors <b>110</b>, <b>112</b> are stored as arrays in memory (not shown) of a scanning system for processing by the A/D converters <b>114</b>, <b>116</b> and the computer <b>124</b>, for example. The size of the pixels may depend on the size and/or number of detector elements comprising the detectors <b>110</b>, <b>112</b>, imaging integration time, etc. The smallest pixel size may correspond to a single detector element or number of detector elements. The size of each detector element and each pixel may be 0.5 cm×0.5 cm, for example.

0050Other X-ray scanning system components known in the art are not shown herein, for ease of illustration. For example, the object <b>106</b> may be conveyed through a shielded tunnel with windows to allow for radiation passage from the sources <b>102</b>, <b>104</b> to the object <b>106</b> and from the object to the detectors <b>110</b>, <b>112</b>. The entire system <b>100</b> may be shielded by concrete or a multilayer shield comprising polyethylene as an inner layer and lead or steel as an outer layer, for example. X-ray scanning systems are described in more detail in U.S. application Ser. No. 11/070,030, U.S. application Ser. No. 11/070,143, and U.S. application Ser. No. 11/070,032, all of which were filed on Feb. 28, 2005, are assigned to the assignee of the present invention, and are incorporated by reference herein.

0051In operation, the object <b>106</b> is moved by the conveying system <b>108</b> in a direction out of the page in the view of <figref idref="DRAWINGS">FIG. 4</figref>. The object <b>106</b> may be moved continuously through the radiation beams <b>103</b>, <b>105</b> or the object may be moved incrementally. As the object <b>106</b> is moved, the first and second sources <b>102</b>, <b>104</b> irradiate the object with radiation. Alternatively, the second source <b>104</b> may only be activated if a suspect region of high density is found after scanning by the first source <b>102</b>. The conveyor <b>108</b> may be reversed for rescanning.

0052<figref idref="DRAWINGS">FIG. 5</figref><i>a </i>is an example of an X-ray image <b>130</b> derived from radiation scanning by the first radiation source <b>102</b> and collected by the first detector <b>110</b>, at a first longitudinal location of the object <b>106</b>. A small, sphere-shaped dense region <b>132</b> is shown. As discussed above, such a dense region could result from scanning of a high density HANM, which could be nuclear material, such as SNM. However, the same dense region <b>132</b> could be caused by a long solid pipe of moderate or low density material, such as iron, for example.

0053To resolve this ambiguity, in this example an image is reconstructed from scanning by the second source <b>104</b> and analyzed. <figref idref="DRAWINGS">FIG. 5</figref><i>b </i>is an example of an X-ray image <b>134</b> of the same object <b>108</b> at the same first longitudinal location of the object, derived from radiation scanning at a second angle by the second radiation source <b>104</b>. In this example, the image <b>134</b> shows an elongated region <b>136</b> of moderate density corresponding to the apparently high density region <b>132</b> of <figref idref="DRAWINGS">FIG. 5</figref><i>a</i>. It can therefore be concluded with a high level of confidence that there is no HANM that may be nuclear material or shielding material, in the object <b>106</b>. The object <b>106</b> can therefore proceed along the stream of commerce.

0054In <figref idref="DRAWINGS">FIG. 5</figref><i>c</i>, which is another example of an X-ray image <b>138</b> of the same object <b>106</b> at the same first longitudinal location, derived from radiation scanning at a second angle, a sphere shaped high density region <b>140</b> of about the same size as the sphere shaped high density region <b>132</b> in <figref idref="DRAWINGS">FIG. 5</figref><i>a</i>, is present. The high density region <b>132</b> is not, therefore, due to the projected density along the line of sight of the radiation beam <b>103</b> through moderate and/or low density material. In this example, it can be concluded with a higher level of confidence that the high density regions <b>132</b>, <b>140</b> in both images correspond to HANM in the object <b>108</b> than if only an image or images at one angle were analyzed. As noted above, that HANM could be nuclear material, such as uranium, or shielding material, such as lead or tungsten, shielding nuclear material. An alarm or other such indicator may be set off automatically or manually, and the object <b>106</b> may be moved to a safe place for further inspection and possible disposal of the object and its contents.

0055The manifest for the object <b>108</b> may then be checked to determine whether an innocuous HANM, such as platinum, gold, silver, or molybdenum, for example (depending on the atomic number where HANM is defined to start in a particular system), has been declared. If not, or regardless, the object <b>108</b> may then be diverted for further testing, such as manual inspections or other non-invasive examinations, such as other radiation scanning techniques. The high density material in the object <b>108</b> and in the image may have other shapes, as well, such as oval, cubic, or rectangular, for example.

0056As mentioned above, the second source <b>104</b> may be activated to generate a radiation beam only if a suspect region is found in an image derived from scanning by the first radiation source <b>102</b>. In this way, the radiation exposure of the object is not increased unnecessarily. Images may then be derived from scanning by the second radiation source <b>104</b> only of the suspect region. Alternatively, scanning may be conducted by the second source <b>104</b> continuously and the entire object may be scanned at both angles.

0057It may be desirable to require that the high density region in an image resulting from scanning by the first source be greater than a minimum size of concern prior to scanning at the second angle or examining an image at the second angle. The minimum size of concern may be a cube with sides of 4.5 cm or a sphere of corresponding volume, for example. In a recent Department of Homeland Security Solicitation for Cargo Advanced Automated Radiography System (CAARS), Solicitation No. HSHQDC-05 R-0007, posted Jan. 10, 2006, available on the Fed Biz Opps Federal Business Opportunities website (http://fbo.gov) at www.fbo.gov/spg/DHS/OCPO/DHS-OCPO/HSHQDC%2DO5%2DR % D0007/listing.html, identification of HANM behind 10 inches (25 mm) of steel having a volume of 100 cubic centimeters, is required. A cube with 4.5 cm walls, for example, has a volume of 91.125 cm³.

0058The images may be readily reviewed by an operator of a scanning system, who may view images from an operator's console (not shown), on the display <b>122</b>, for example. High density regions in images resulting from scanning at the first angle can be readily identified visually, based on the contrast of regions of the image. High density regions may appear to be darker or lighter than the surrounding portions of an image, depending on how the detected radiation is processed into the image. Upon identification of such high density regions, the operator may initiate scanning at the second angle of the same longitudinal location as the first image or through the portion of the object corresponding to the high density region in the first image. The results may again be displayed and visually analyzed and compared on the display <b>122</b>.

0059The images or the data underlying the images may be automatically analyzed as well, by comparing detected radiation intensities to a function, such as a threshold, for example. The function may be based on a predetermined atomic number or density at or above which it is desired to identify an HANM. In one example, an image density threshold is established, when an X-ray system <b>100</b> is calibrated on site. Thresholds may be established for each radiation angle. Threshold generation is discussed further, below.

0060Images or data from scanning at one angle may then be automatically analyzed to identify regions with intensities indicative of densities that meet or exceed the density threshold. Regions that do not exceed the threshold do not have atomic numbers and densities high enough to potentially correspond to nuclear materials or shielding materials and in this example are not of concern.

0061If only nuclear materials and shielding materials greater than a certain size are of concern, a size threshold may also be created. The size of the high density region may be automatically determined by counting contiguous pixels having intensities greater than the threshold, discussed above, for example. High density regions may be automatically identified by comparing the intensity of image pixels or groupings of pixels to the predetermined threshold based on the calibration discussed above, as is known in the art.

0062As discussed above, in one example, HANMs are defined as materials having an atomic number greater than a predetermined atomic number. The predetermined atomic number may be determined based on the materials that it is desired to identify. For example, uranium (Z=92) has the lowest atomic number of the SNMs. To be sure to identify at least potential SNMs, the predetermined atomic number may be 92 or less. To also be able to identify the presence of the shielding materials lead (Z=82) and tungsten (Z=74), the predetermined atomic number may be 74 or less. Since radiation attenuation/transmission is related to the density of the materials along the path of the radiation through the object, density may be used to define a function, such as a threshold, to differentiate HANM from non-HANM based on the detected radiation. HANM may be defined in terms of density as well.

0063<figref idref="DRAWINGS">FIG. 6</figref> is a graph of Physical Density (g/cm³) versus Atomic Number for metals up to an atomic number of 57 (indicated by black points) and certain other elements. The physical density of lead (atomic number 82) of about 11 g/cm³is indicated by an open triangle “L”. The physical densities of uranium and plutonium are indicated by Circle <b>1</b> and Circle <b>2</b>, respectively. Uranium has a density of about 19.5 g/cm³and plutonium has a density of about 19.7 g/cm³. The physical densities of uranium and plutonium are nearly twice that of lead and the other metals.

0064Lead (Z=82) has the lowest density of materials of interest in this example. To provide sufficient margin to be sure to identify lead, a density threshold may be based on a density of 10 g/cm³. However, as shown in <figref idref="DRAWINGS">FIG. 6</figref>, molybdenum (atomic number 42, density 10.2 g/cm³), rhodium (atomic number 45, density 12.4 g/cm³), palladium (atomic number 46, density 12.0 g/cm³), and silver (atomic number 47, density 10.5 g/cm³) have comparable densities. Of these metals, only silver is shipped in significant amounts and when it is, it is typically identified on a manifest. Therefore, in actual commerce, the presence of high density, non-nuclear, non-shielding materials should not cause many nuisance alarms. Other thresholds may be selected as well.

0065The material densities in <figref idref="DRAWINGS">FIG. 6</figref> are three dimensional values having units of g/cm³, for example. When scanning at each angle, however, the detected radiation at each angle is only indicative of the radiation transmitted through or attenuated by an “effective,” “projected,” or “line of sight” density, which is a two dimensional value having units of g/cm², for example. For this reason, the radiation detected after passing through a small amount of uranium and a large amount of iron along the line of sight of the radiation beam, may be the same, as may the resulting radiation images. Without being limited to any theory of operation, embodiments of the invention scan at a second angle to obtain information about the material contents of the container along the line of sight of the radiation beam at the first angle, in the third dimension.

0066In one example, a threshold corresponding to 10 g/cm³, discussed above with respect to <figref idref="DRAWINGS">FIG. 6</figref> (or any other selected density), may be determined by calculating the effective density of a material having a density of 10 g/cm³, along a line of sight of the smallest HANM of concern. For example, if it is desired to identify HANM of a cube having sides of 4.5 cm or a sphere having a diameter of 4.5 cm, for example, the threshold may be calculated by multiplying the actual material density (10 g/cm³) by the length or diameter of the object of concern (4.5 cm), which yields a two dimensional projected density threshold of 45 g/cm².

0067The scanning system <b>100</b> may be calibrated to the desired two dimensional projected density threshold by scanning a cube of a material having the same effective density as the threshold at one or both scanning angles. A cube having sides of 45 cm, for example, which has a density of 1 g/cm³, has an effective density of 45 g/cm. Cube sizes of other materials, such as aluminum and steel, may be readily calculated, as well. The detected radiation and/or the image densities in resulting images may then be used as thresholds.

0068According to the Department of Homeland Security Solicitation for Cargo Advanced Automated Radiography System (CAARS), Solicitation No. HSHQDC-05 R-0007, posted Jan. 10, 2006 identified further, above, a radiation scanning system should identify a material with an atomic number greater than 72 as a HANM. Those specifications also require that materials having an atomic number less than 57 not be considered HANM. Hafnium, which has an atomic number of 72, has a density of about 13.2 g/cm³. A threshold to identify HANM above an atomic number of 72 having a size of a 4.5 cm cube, or other size, may be readily defined based on the density of hafnium, for example, as described above. The density of tantalum (Z=73), which has a density of 16.6 g/cm³, could be used, as well.

0069As discussed above, materials with an atomic number between 56 and 72 are rarely transported in commerce and when they are, they are typically declared on a manifest. High atomic number, high density limits defining HANM as materials with atomic numbers greater than 72, for example, will include as HANM nuclear materials and shielding materials. Most commonly shipped metals, such as tin (Z=50), silver (Z=47), molybdenum (Z=42), zinc (Z=30), copper (Z=29), nickel (Z=28), cobalt (Z=27), and iron (Z=26) would not be identified as HANM. It is noted that cobalt (Z=27) and carbon (Z=6) have radioactive isotopes (cobalt-60, carbon-12), which would not be identified as an HANM if the predetermined atomic number is greater than 27. However, since radioactive materials would probably be transported within lead or tungsten shielding material, its presence could be identified by detecting the presence of the high atomic number shielding material.

0070Since iron (Z=26) is the highest atomic number material commonly found in cargo conveyances and luggage, the threshold detection may be based on iron. To avoid nuisance alarms caused by the presence of iron, in this example, HANM could be defined as materials having an atomic number greater than 26. Copper (Z=29) and nickel (Z=28), which have atomic numbers close to iron, could also be used in a similar way.

0071The threshold may also be established by scanning a piece of test material having a predetermined atomic number or density, in the scanning system <b>100</b> at each angle, detecting the radiation interacting with the piece, and processing the detected radiation. If HANM are defined in a particular system to be materials having an atomic number greater than iron (Z=26), for example, a piece of iron could be used. The test material may be in a shape of a cube having a size of the smallest SNM of concern. For example, the test piece may be a cube having sides of 4.5 cm. The function may be the same or may be different for the two scanning angles, depending on the state and position of the system components, such as the sources <b>102</b>, <b>104</b> and the detectors <b>110</b>, <b>112</b>.

0072<figref idref="DRAWINGS">FIG. 7</figref> is an example of a method <b>200</b> for examining objects in accordance with an embodiment of the invention. An object is scanned at a first angle, in Step <b>202</b>. Detected radiation is analyzed, in Step <b>204</b>. It is then determined whether the detected radiation is indicative of the presence of a first high density region in the object, in Step <b>206</b>. This may be determined automatically by a processor <b>118</b>, such as the computer <b>124</b>, configured to analyze an image or data underlying the image by comparison to a density threshold, for example. If not, it is determined that there is no HANM present, in Step <b>208</b>.

0073If the detected data is indicative of a high density region in the object, in this example it is determined whether the size of the region is greater than a predetermined size, in Step <b>210</b>. This may also be determined automatically by the processor <b>118</b>, such as the computer <b>124</b>, by comparison to a diameter or area threshold. If not, then it is concluded that high density HANM is not present, in Step <b>212</b>.

0074If the high density region is greater than the predetermined size, in this example, at least a portion of the object at the same longitudinal location of the object corresponding to the first high density region, is scanned at the second angle, in Step <b>214</b>. The detected radiation is analyzed, in Step <b>216</b>, and it is determined whether the detected data is indicative of a second high density region corresponding to the location of the first high density region in the object, in Step <b>218</b>. As above, this may be done automatically.

0075If the detected data is not indicative of a second high density region corresponding to the location of the first high density region in the object, then it is determined that an HANM is not present, in Step <b>220</b>.

0076If the detected data is indicative of a second high density region corresponding to the location of the first high density region, then it is concluded that HANM is at least potentially present, in Step <b>222</b>. A visual or audible alarm may be automatically initiated by the processor <b>118</b>/computer <b>124</b>, if HANM is found. The manifest may then be checked to determine whether non-nuclear HANM, such as silver (Z=47), in this example, has been declared. This may also be done automatically by the processor/computer, if an electronic manifest is provided, or by an operator. If not, or regardless, the object may be removed from the conveyor <b>108</b> for further testing, such as manual inspection and/or additional radiation scanning, as discussed further below.

0077The processor <b>118</b>/computer <b>124</b> may be programmed in software and/or hardware to automatically analyze the images and/or data, and to control operation of the system <b>100</b>. In one example, the methods of embodiments of the invention may be implemented through an Application Specific Integrated Circuit (ASIC), for example.

0078Steps <b>206</b> and <b>218</b> may also be performed visually by an operator viewing images of the object. If visual inspection of images is conducted, the size check in step <b>210</b> may optionally be skipped.

0079<figref idref="DRAWINGS">FIG. 8</figref> is a schematic representation of another example of a radiation scanning system <b>300</b> in accordance with another embodiment, in which a radiation source <b>302</b> is movably supported on a rail <b>304</b>. An object <b>306</b> is shown supported by a conveyor <b>308</b> that moves the object out of the page in this view. The source <b>302</b> is shown in a first position A for scanning at a first angle. The source <b>302</b> is movable to a second position, such as position B (shown in phantom), for scanning at a second angle. Other components of the system <b>300</b>, such as those shown in <figref idref="DRAWINGS">FIG. 4</figref> and discussed above, as well as others known in the art, are not shown in this view to ease illustration.

0080The source <b>302</b> may be moved along the rail <b>304</b> by chains (not shown) driven by a motor (not shown), for example. Two detectors <b>310</b>, <b>312</b> are also shown. The source <b>302</b> may be coupled to the rail via a hinge <b>314</b> so that the source may be rotated into a desired alignment with the object <b>306</b> in the different positions A, B.

0081The rail <b>304</b> is arcuate, such as semi-circular, in this example, but it may have other configurations. For example, it may be straight. The rail <b>304</b> may extend somewhat more than one-quarter circle, to enable scanning at positions up to about 90° apart, or to somewhat more than about 135°, for example.

0082If a suspect region is found by scanning in the first position A, the operator can cause the source <b>302</b> to be moved to the second position B and initiate scanning. A processor, such as the computer <b>124</b>, may be programmed to automatically identify potential HANM based on detected radiation, as discussed above, and may also be programmed to automatically advance the source <b>302</b> to the second position B and initiate scanning.

0083In one example, the positions A and B are predetermined and the source <b>302</b> is only movable between those positions. The second position B may be any position enabling scanning at a second angle forming an angle α with the first beam in the ranges discussed above.

0084In another example, the source <b>302</b> is movable from a first to one or a plurality of second predetermined positions along the rail, or may be moved to any desired position between positions A and B. In this example, an operator may determine the second angle to scan an object. If the image or data collected at the second angle is not sufficient to determine whether the object is likely to contain HANM, the source <b>302</b> may be moved to another position.

0085A radiation source <b>402</b> and a detector <b>404</b> may also be movably supported on a circular gantry <b>406</b>, as shown in <figref idref="DRAWINGS">FIG. 9</figref>. An object <b>408</b> supported by a conveyor <b>410</b> that moves the object out of the page in this view, are also shown. The gantry <b>406</b> may be rotatable clockwise or counterclockwise, here counterclockwise, to position the source <b>402</b> and the detector <b>404</b> in a first position A and a second B (shown in phantom). As above, the positions A and B may be determined or may be set by an operator. The gantry <b>406</b> may move the source <b>402</b> and the detector <b>404</b> to different and/or additional positions. Other components of the system <b>400</b>, such as those shown in <figref idref="DRAWINGS">FIG. 4</figref> and discussed above, as well as others known in the art, are not shown in this view to ease illustration. The source <b>402</b> and the detector <b>404</b> may also be supported by a C-arm connected to a rotating gantry. Such a C-arm type scanning system is described in U.S. Patent Publication No. US 2004/0068169 A1, which was published on Apr. 8, 2004, is assigned to the assignee of the present invention, and is incorporated by reference herein.

0086<figref idref="DRAWINGS">FIG. 10</figref> is a schematic representation of another example of a radiation scanning system <b>500</b> in accordance with another embodiment of the invention, in which a radiation source <b>502</b> comprises a source of charged particles (not shown) coupled to an accelerator body <b>503</b> and two shielded targets <b>504</b>, <b>506</b> to generate radiation to irradiate an object <b>530</b> at two angles. The object is moved to the left, as indicated by the arrow L, by a conveyor (not shown). The detectors are also not shown in this view, to ease illustration. Other components of the system <b>500</b>, such as those shown in <figref idref="DRAWINGS">FIG. 4</figref> and discussed above, as well as others known in the art, are not shown in this view to ease illustration.

0087The shielded targets <b>504</b>, <b>506</b> comprise target material <b>504</b><i>a</i>, <b>506</b><i>a </i>(shown in phantom) surrounded by shielding material <b>505</b>, <b>507</b>. The shielding material <b>505</b>, <b>507</b>, which may be tungsten, for example, defines slots <b>505</b><i>a</i>, <b>507</b><i>a</i>, respectively, extending from the targets <b>504</b><i>a</i>, <b>506</b><i>a </i>to the exterior of the shielding material, to define a radiation beam at a desired angle and having a desired shape, directed towards the object <b>530</b>. A fan beam or a cone beam may be defined, for example. Such a shielded target is described in more detail in U.S. application Ser. No. 10/199,781, which was filed on Jul. 19, 2002, was published on Mar. 25, 2004 as U.S. Publication No. 20040057554 A1, is assigned to the assignee of the present invention, and is incorporated by reference herein.

0088An electron beam from the linear accelerator body <b>503</b> is selectively directed to one of the two shielded targets <b>504</b>, <b>506</b> by an electromagnetic bend magnet <b>508</b>. A first drift tube <b>510</b> extends from the output end <b>511</b> of the linear accelerator body <b>502</b> to the bend magnet <b>508</b>. Two drift tubes <b>512</b>, <b>514</b> extend at right angles from the bend magnet <b>508</b>, to the two shielded targets <b>504</b>, <b>506</b>.

0089The two shielded targets <b>504</b>, <b>506</b> are positioned to irradiate the object <b>530</b> at two different angles. The targets <b>504</b>, <b>506</b> may irradiate the object <b>530</b> at two different longitudinal positions. The detectors (not shown) may be straight or L-shaped detectors, as above. In operation, the electromagnetic bend magnet <b>508</b>, which is a well known device, is used to deflect the electron beam into one or the other tube <b>512</b>, <b>514</b> as the object <b>530</b> is conveyed through the scanning unit. In accordance with an embodiment of the present invention, the magnet <b>508</b> may deflect the election beam towards the shielded target <b>506</b> for scanning at a first angle, and if a suspected HANM is identified, the magnet <b>508</b> may be automatically or manually switched to deflect the electron beam towards the shielded target <b>504</b> for scanning at a second angle, as described above. Alternatively, the magnet <b>508</b> may be automatically switched to alternately deflect the electron beam to the first and second shielded targets <b>504</b>, <b>506</b>, as the object <b>530</b> is conveyed through the system <b>500</b>.

0090Depending on space constraints in the configuration of the scanning unit, it may be advantageous to align the linear accelerator body <b>502</b> with one of the shielded targets. <figref idref="DRAWINGS">FIG. 11</figref> is a perspective view of an X-ray source <b>600</b>, comprising a linear accelerator body <b>602</b> that is aligned with a first shielded target <b>604</b>. A first drift tube <b>606</b> couples the open end <b>608</b> of the linear accelerator body <b>602</b> to a first bend magnet <b>610</b>. A second drift tube <b>612</b> couples the first bend magnet <b>610</b> to the first shielded target <b>604</b>. A third drift tube <b>612</b> couples the first bend magnet <b>610</b> to a second bend magnet <b>614</b>. A fourth drift tube <b>616</b> couples the second bend magnet <b>614</b> to a second shielded target <b>618</b>. The first bend magnet <b>614</b> selectively allows the electron beam to pass to the first shielded target <b>604</b> or deflects the electron beam to the second shielded target <b>618</b>. The first bend magnet <b>604</b> is an electromagnet. In this case, the second bend magnet <b>614</b>, which may always be on, may be a permanent magnet or an electromagnet.

0091The method and system of the present invention may be combined with other examination methods and systems. For example, a first examination of the container may be conducted by a dual energy technique. The second radiation source <b>104</b> may be at a different energy than the first radiation source <b>102</b>, a third radiation source (not shown) may be provided to generate radiation at a different energy, or the first or second radiation source <b>102</b>, <b>104</b> may be switched between energy levels. Dual energy techniques are discussed in U.S. application Ser. No. 11/070,030, U.S. application Ser. No. 11/070,143, and U.S. application Ser. No. 11/070,032, all of which were filed on Feb. 28, 2005, are assigned to the assignee of the present invention, and are incorporated by reference herein (“the Dual Energy Applications”). Dual energy techniques are also discussed in U.S. Pat. No. 5,524,133, which is discussed above and is incorporated by reference herein. If a suspected HANM is found by the dual energy technique, then the suspected HANM may be analyzed at the first and second angles. False alarms may thereby be further reduced, without reducing sensitivity.

0092Alternatively, the dual view technique discussed herein may be a first test. If potential HANM is found, the dual energy tests discussed in the Varian Applications, above, may be conducted to determine whether the potential HANM is an HANM, with a higher level of confidence. Another confirmatory test described in the Dual Energy Applications involves the detection of delayed neutrons, which are indicative of the presence of nuclear material, to verify the dual energy, or other tests. The delayed neutron test may also be used with the dual view test described herein, with or without the dual energy tests described in the Varian Applications.

0093Since the dual angle test is simpler than many others tests, it may be performed first.

0094While the invention is particularly suited for scanning cargo conveyances for HANM, the invention may be readily adapted to scan other objects, such as luggage and carry-on bags in airports and seaports, as well.

0095While in the embodiments described above, the object is moved by a conveyor system through radiation beams generated by radiation sources that are stationary during scanning, the object could be stationary and the radiation sources could be moved across the length of the object. In addition, an object could be moved through the radiation beams by a crane or other such device. For example, the object could be scanned while being moved horizontally or vertically through radiation beams generated by sources mounted on or near the crane. The crane may be loading or unloading cargo containers between a ship and a truck, for example, as described in U.S. application Ser. No. 10/356,101, which was filed on Jan. 31, 2003 and was published as U.S. Publication No. 20040156477 A1 on Aug. 12, 2004, and U.S. application Ser. No. 11/203,491, which was filed on Aug. 12, 2005, and was published as 20060115043 A1 on Jun. 1, 2006, both of which are assigned to the assignee of the present invention and are incorporated by reference herein.

0096In addition, while an X-ray source or sources are described in the examples above, the source or sources may provide other types of radiation, such gamma rays, for example.

0097One of ordinary skill in the art will recognize that other changes may be made to the embodiments described herein without departing from the spirit and scope of the invention, which is defined by the claims, below.